Strict requirements must be met for suppression of machine produced noise in the enviroment. One important source of noise consists of the intake and exhaust air pipes for ventilation in connection with various industrial plants and other large buildings. This is especially true for blower noise, which is dispersed into the environment through the air ducts. Blowers are usually chosen or selected on the basis of the quantity of air produced by them, and attention is frequently not paid to the noise produced by them. Further, the installation and ducting has a large impact on the quality and quantity of noise emitted with the environment. The noise range produced by the blowers has quite a wide spectrum, which also imposes particular requirements on the noise suppression, in that it must be able to reduce the intensities of acoustic vibrations across this broad range.
Paper mills are particularly demanding with respect to suppression of noise, because the ventilation of the paper machine hall, and in particular the elimination of moisture from the drying section of the paper machine, require large quantities of air. It is a particular problem in the air-conditioning of a paper machine, particularly with respect to exhaust of air from the wire part. In that case, the exhaust air contains a large amount of moisture and also fibers and various paper fillers, such as kaolin clay. Such solid materials tend to block the perforated plates in an absorptive sound attenuator, and the condensing moisture wets the absorption materials and deteriorates their sound absorption properties.
The prior-art sound attenuators in general, and the sound attenuators used in paper machine ventilation in particular, have been difficult to maintain, because their absorption material is frequently subject to high moisture conditions and is susceptible to becoming wet, and the absorption material is difficult to cleanse and to replace. Consequently, absorptive sound attenuators which have been designed and dimensioned to initially operate efficiently, over time tend to operate unsatisfactorily because they are "blocked" as a result of water and/or impurity accumulation.
Since the noise produced by blowers has a wide acoustic spectrum, it is frequently necessary to use both absorptive and reactive sound attenuators in the intake and exhaust air ducts connected to the blowers. Absorptive sound attenuators operate primarily at higher frequencies; the maximum of their attenuation is at a frequency of about 1000 Hz, whereas reactive sound attenuators, which are mainly based on various resonator constructions, operate most efficiently at low frequencies, and their maximum attenuation is, as a rule, tuned in a range of about 100 to 200 Hz.
One important property of sound attenuators connected to the input or output ducts of blowers, besides the attenuation of noise, is the pressure drop or flow impedance inherent in their design.